A physical unclonable function (PUF) is a silicon die-specific random function or a silicon biometric that is unique for every instance of the die. In general, PUFs derive their randomness from uncontrolled random variations in the integrated circuit fabrication process (usually undesirable) to create practically unclonable functions even if the original design files are compromised. PUFs can be used as building blocks in many secure systems for applications such as device identification/authentication and secret key generation. PUFs can be an alternative to storing of random secret bits in volatile or non-volatile memory (which may be vulnerable to attacks) by instead generating these random bits every time the PUFs are activated.
PUF implementations generate random bits by amplifying some electrical characteristic (e.g., delay or threshold voltage) from two nominally identical circuit components in the PUF array. These electrical differences, especially when small, often flip polarity across environmental variations (e.g., voltage and temperature), in the presence of ambient noise, or over device aging, resulting in some bits of the raw PUF response being inconsistent/unreliable. For some designs, greater than 25% of the PUF response bits may be unreliable across environmental variations. Because electrical differences of larger magnitude require larger variations to flip polarity, a PUF bit may be more reliable when generated by amplifying a larger electrical difference.
Although some applications, such as identification and authentication, can be designed to tolerate a few errors in the response without significant loss of security, all applications can benefit from more reliable PUFs, and applications such as key generation may require the PUF response to be perfectly reliable.